Secondly, you need your own relationship with the good customers so as to provide a backup if the salesperson leaves. In the worst-case scenario, if a disgruntled salesperson leaves and joins the competition, you need to know who the customers are, and they need to know that you are the face of the company behind the front-line salesperson.
Notice that the emphasis here is on “good” customers. I don’t think that you need to know every customer, nor do you need to know the prospects.
Now, back to the question.
How often should a sales manager visit the customers? Often enough to accomplish the above two objectives. These are visits that you make, by yourself, without the salesperson present.
Then, you should visit them with your salespeople to support the salesperson, add credibility to his/her presence, and to coach and counsel the salesperson on techniques and strategy.
That’s the first answer. The second answer is simpler.
How much time should you spend in the field, visiting customers and working with salespeople?
Answer: More than you do. I have yet to meet a sales manager who spent as much time in the field as he/she would like to spend. I can almost categorically state that every sales manager should spend more time in the field than they do.